target/riscv: Add MXL/SXL/UXL to TB_FLAGS

Begin adding support for switching XLEN at runtime.  Extract the
effective XLEN from MISA and MSTATUS and store for use during translation.

Reviewed-by: LIU Zhiwei <zhiwei_liu@c-sky.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20211020031709.359469-6-richard.henderson@linaro.org
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>